Chromosome segregation during meiosis places one chromosome, either the maternal or paternal copy, in each gamete. Accurate segregation of genetic material requires that the parental chromosomes interact with one another and exchange genetic information through the formation of crossovers. Failure to make crossovers leads to errors in meiotic chromosome segregation, and consequently to infertility and congenital birth defects. My ESI-MIRA award funds research that addresses the mechanisms of chromosome interaction and segregation. The molecular mechanisms that allow chromosomes to form, regulate, and respond to crossovers are poorly understood. The main project in the lab explores how the Synaptonemal Complex (SC)?a conserved structure that assembles between the parental chromosomes and regulates the distribution of crossovers?implements chromosome-wide regulation. We have developed a novel mutagenesis and genetic screening strategy to isolate separation-of-function mutations that perturb specific properties of the SC. The mutants we isolated so far allowed us to define a domain that regulates how the SC brings together the maternal and paternal chromosomes and aligns them. While we have already learned a great deal about our mutants through cytology and genetics, how these mutations impact the SC ultrastructure or the organization of the SC subunits remains unknown. The dimensions of the SC (~100nm) cannot be resolved by confocal microscopy. In the request for an Administrative Supplement, I am asking for a super-resolution microscopy system. This system works similarly to a confocal microscope, but can deliver resolutions as high as 30nm. This enhanced resolution will allow us to probe the ultrastructure of the SC in our mutants, while maintaining contextual knowledge on chromosome morphology and the localization of different subunits. The information we will obtain will allow us to link the ultrastructure of the SC to the functions it carries out.